Directional filter

ABSTRACT

A directional filter including a primary waveguide, a secondary waveguide, each of the waveguides having a rectangular crosssectional area, resonator means having a circular cross-sectional area and opposite ends along which the primary and secondary waveguides are arranged respectively, the resonator means resonating at either transverse electric mode TEN11 or transverse magnetic mode TMN10 (N 1, 2, . . .), a first directional coupler comprising at least two coupling elements distributed between the primary waveguide and one of the opposite ends of the resonator means for coupling the primary waveguide and the resonator means, and a second directional coupler comprising at least two coupling elements distributed between the secondary waveguide and the other of the opposite ends of the resonator means for coupling the secondary waveguide and the resonator means.

United States. Patent [191 Shimada et al.

Oct. 1, 1974 DIRECTIONAL FILTER Japan [73] Assignees: Nippon Telegraph and Telephone Public Corporation, Tokyo; Hitachi Electronics Co., Ltd., Chiyoda-ku, both of, Japan [22] Filed: Dec. 13, 1972 [2]] Appl. No.: 314,518

[30] Foreign Application Priority Data Dec. 15,1971 Japan 46-101012 [52] US. Cl. 333/73 W, 333/10 [5l] Int. Cl. H0lp 1/20 [58] Field of Search 333/10, 73 W [56] References Cited UNITED STATES PATENTS 2,936,430 5/1960 Marie 333/10 2,939,093 5/1960 Marie 333/10 3,042,883 7/]962 Pannenborg et al 333/10 OTHER PUBLICATIONS Matthaei et al., Microwave Filters, Impedance-Matching Networks, and Coupling Structures, McGraw-Hill, N.Y., 1964, TK3226M38, pp. 856 and 859 relied on.

Primary Examiner-Paul L. Gensler Attorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT A directional filter including a primary waveguide, a secondary waveguide, each of the waveguides having a rectangular cross-sectional area, resonator means having a circular cross-sectional area and opposite ends along which the primary and secondary waveguides are arranged respectively, the resonator means resonating at either transverse electric mode TE or transverse magnetic mode TM (N l, 2, a first directional coupler comprising at least two coupling elements distributed between the primary waveguide and one of the opposite ends of the resonator means for coupling the primary waveguide and the resonator means, and a second directional coupler comprising at least two coupling elements distributed between the secondary waveguide and the other of the opposite ends of the resonator means for coupling the secondary waveguide and the resonator means.

4 Claims, 9 Drawing Figures PATENIEBUCH m4 sum 10F FIG.4

B d O 2 B d m y 3 wmv PAIENTEU 3.839.688

sum 2 or 2 2A PRIOR ART PRIOR ART DIRECTIONAL FILTER FIELD OF THE INVENTION The present invention relates to a directional filter adapted to be used as a channel branching filter for microwave, quasi-millimeterwave and millimeterwave frequency bands.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially exploded perspective view of a conventional directional filter.

FIGS. 2A and 2B are diagrams showing partially exploded views of the resonator provided in the directional filter of FIG. 1.

FIG. 3 is a partially exploded perspective view showing an embodiment of the directional filter according to the present invention.

FIG. 4 is a perspective view showing another embodiment of the directional filter according to the present invention.

FIGS. 5A and 5B are partially exploded perspective views showing the resonator provided in the directional filter according to the present invention shown in FIGS. 3 and 4.

FIG. 6 is a diagram showing the frequency characteristics of the directional filter according to the present invention.

FIG. 7 is a partially exploded perspective view showing another embodiment of the directional filter according to the present invention.

DESCRIPTION OF THE PRIOR ART A typical conventional directional filter for microwave, quasi-millimeterwave and millimeterwave frequency bands has a construction as shown in FIG. 1. The directional filter includes a resonator 5' which, as shown in FIGS. 2A and 2B, comprises conductive cylinders' 1 and 2 arranged concentrically with each other and annular conductive end plates 3 and 4 extending from the one cylinder to the other cylinder. The resonator 5 has an annular cross-sectional area and a main resonance frequency of f,,. A primary waveguide 6' and a secondary waveguide 7' which have rectangular cross-sectional areas, are bent and folded in semicircular forms and extend in opposite directions are mounted on the end plates 3 and 4 of the resonator 5' in such a manner that the folded portions of face plates 8 and 9 of the primary and secondary waveguides are coupled with the end plates 3 and 4, respectively. A plurality of coupling holes 10 acting as a directional coupler are bored through the end plate 3 of the resonator 5' and the adjoining face plate 8 of the primary waveguide 6' coupled with the end plate 3, while a plurality of coupling holes 11 acting as a directional coupler are formed through the end plate 4 of the resonator 5 and the adjoining face plate 9 of the secondary waveguide 7' coupled with the end plate 4. Actually, however, that portions of the face plates 8 and 9 which are to be coupled with the end plates 3 and 4 may be doubled by' the end plates 3 and 4.

Assuming that a multiplied signal (F +F,+F F,) of signals F F F F, with center frequencies of f 1",, f 1", respectively are applied to one end 12 of the primary waveguide 6, only the signal F with the center frequency of f is resonated by the resonator and taken out of one end 13 of the secondary waveguide 7, the remaining signals F F F, being guided out by way of the other end 14 of the primary waveguide 6. Therefore, it is possible by using this directional filter, to multiply or separate a plurality of signals of different center frequencies, thereby constituting a channel filter.

In order to set the main frequency )1, in this conventional directional filter, the length of the circumference of the circle having a radius equal to the average radius of the annular cross-sectional area of the resonator 5' is set at a value which is N times (N l, 2, as large as the wavelength in the resonator 5 corresponding to the frequency f Thus the resonator 5' has the resonance order of N but also has adjacent resonance frequenciesf andfou of the (N l)th and (N l)th orders respectively. These adjacent resonance frequencies must not be present within the frequency band occupied by the signal (F F F If the resonance frequency f exists within the frequency band occupied by the signal (F F F for example, a signal component within this band having the same frequency as the frequency f is undesirably taken out of the end 13 of the secondary waveguide 7' together with the signal component having the center frequency f,,, thereby making it impossible to achieve the required filter function.

Solution of this problem requires the reduction in the resonance order N of the resonator 5'. If the resonance order Nof the conventional directional filter of FIG. 1 is to be reduced, however, the size of the waveguides 6' and 7' as well as the resonator 5 coupled therewith must be correspondingly reduced with the result of the difficult manufacture of these particular components. It is especially difficult to couple the inner conductive cylinder 2 of the resonator 5' mechanically with the end plates 3 and 4 without any adverse effect upon smooth electrical contact therebetween. For example, if a directional filter is adapted to a millimeterwave frequency band, the diameter of the inner cylinder 2 must be made less than about 1 mm in the resonance fre quency of 50 Gl-lz and the resonance order of N 2 so that the integeral construction of the cylinder 2 and the end plates 3 and 4 becomes impossible. Also, even if individually prepared cylinder 2 and end plates 3 and 4 are constructed integrally it results in worse contact between the cylinder 2 and the end plates 3 and 4 and hence in increased thermal loss and lower Q of the resonator. Further, the small diameter of the cylinder 2 tends to contribute to the distortion thereof and hence to that generation of wave in the opposite direction which leads to worse directional action and lower VSWR (voltage standing-wave ratio).

Furthermore, from the fact that a relatively large curent may flow in the neighborhood of the cylinder 2 by reducing the diameter to the inner cylinder 2 to reduce the resonance order N of the resonator 5, it is apparent that the unloaded Q of the resonator 5 is reduced, thereby increasing loss due to the insertion of the filter. Thus, the reduction of resonance order N has some limitations which in turn impose restrictions on the filter function over a wide frequency band.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a unique directional filter which obviates all the disadvantages of the above-described conventional directional filter.

The directional filter according to the present invention includes a primary waveguide, a secondary waveguide, each of said waveguides having arectangular cross-sectional area, resonator means having a circular cross-sectional area and opposite ends along which said primary and secondary waveguides are arranged respectively, said resonator means resonating at either transverse electric mode TE or transverse magnetic mode TM (N I, 2, a first directional coupler comprising at least two coupling elements distributed between said primary waveguide and one of the opposite ends of said resonator means for coupling said primary waveguide and said resonator means, and a second directional coupler comprising at least two coupling elements distributed between said secondary waveguide and the other of the opposite ends of said resonator means for coupling said secondary waveguide andsaid resonator means. The use of the resonator whose cross-sectional area is circular eliminates an inner conductive cylinder of the conventional directional filter comprising a resonator whose crosssectional area is annular, and enables the manufacturing of the resonator with higher accuracy, the higher mechanical stability of the resonator and the setting of the center frequency with higher accuracy.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be explained with reference to FIGS. 3, 5A and 5B. A resonator 5 used in the directional filter according to the present invention, as shown in FIGS. 5A and 5B, comprises a conductive cylinder 17 having the ends thereof sealed with conductive end plates and 16 has a circular cross-sectional area. As shown in FIG. 3, a primary waveguide 6 and a secondary waveguide 7 which have rectangular cross-sectional areas, are bent and folded in semicircular forms and extend in opposite directions are arranged on the end plates 15 and 16 in such a manner that the folded portions of face plates 18 and 19 of the primary and secondary waveguides are coupled with the end plates 15 and 16, respectively. The waveguide 6 has two ways which include a common face plate 20 and the waveguide 7 has two ways which include a common face plate 21, the common face plates 20 and 21 extending up to the central axis of the resonator 5 but in opposite directions to each other. Face plates 32 and 33 of the waveguide 6 positioned opposite to the common face plate 20 and face plates 22 and 23 of the waveguide 7 positioned opposite to the common plate 21 are respectively bent in semicircular forms along the circumference of the cylinder 17 of the resonator 5. A plurality of slits 24 which act as a directional coupler and extend radially about the center of the end plate 15 .of the resonator 5 are bored through both the end plate 15 and the face plate 18 of the primary waveguide 6 coupled therewith, while a plurality of slits 25 which act as a directional coupler and extend radially about the center of the end plate 16 of the resonator 5 are formed through both the end plate 16 and the face plate 19 of the secondary waveguide 7 coupled therewith. In practice, the end plates 15 and 16 of the resonator 5 maydouble respectively as portions of the face plates 18 and 19 which are to be coupled with the end plates 15 and 16.

When a multiplied signal (F F, F F,) of signals F F F F, with center frequencies of f0, f f 1",, respectively, is applied to one end 26 of the primary waveguide 6 as in the case of FIG. 1, the multiplied signal is excited by the resonator 5 through the slits 24 if the resonator 5 is of such a size that it has the resonance order of N (N I, 2, and resonates at the center frequency of f under the cylinder-shaped transverse electric resonance mode TE (for the resonator of FIG. 5A) or cylinder-shaped transverse magnetic resonance mode TM (for the resonator of F IG. 5B), so that only the signal F which has been selected by the resonator 5 is taken out of one end 27 of the secondary waveguide 7 through the slits 25, the remaining signals F F F, being led out of the other end 28 of the primary waveguide 6 thereby achieving the same characteristics as those of the conventional directional filter. Reflection of waves which may occur due to the sharp curve in the waveguides 6 and 7 can be prevented by providing posts 29 and 30 of metal or dielectric material inside of the waveguides 6 and 7.

It will be understood from the above description that the same characteristics as those of the conventional directional filter are achieved by constructing the directional filter according to the present invention such that it comprises a'cylindrical resonator 5 which resonates under the transverse electric mode TE or transverse magnetic mode TM Also, the the resonator according to the present invention has no restrictions as those to which the conventional resonator of FIGS. 1 and 2 are subjected, thus making it practicable to achieve the filter function without any reduction in unloaded O which otherwise might occur due to unsatisfactory contact of the inner cylinder. As an example, the diagram of FIG. 6 illustrates the transmission lossfrequency characteristics-of the directional filter employing the cylinder-shaped transverse magnetic resonance mode TM which characteristics were measured by the use of a sweep oscillator. In this diagram, curve 38 shows transmission loss between the input end 26 and output end 27 of the waveguides, curve 36 shows transmission loss between the input 26 and output 28 and curve 37 shows transmission loss between the output ends 27 and 28, the frequency band for transmission loss 3dB at the center frequency 51.05 GI-Iz .being 680 MHz and the transmission loss (or branch loss) between the input end 26 and output end 27 of the waveguides at the center frequency being 0.8dB which is almost identical with the characteristics of the conventional directional filter.

In place of the above-mentioned embodiment in which each of the primary and secondary waveguides is folded in opposite directions, the directional filter according to the present invention may be constructed as illustrated in FIG. 4. In the embodiment of FIG. 4, a primary waveguide 6a is coupled with the cylindrical resonator 5 with the transverse magnetic resonance mode TM through a directional coupler 24 comprising a plurality of slits, while a secondary waveguide 7a is connected with the resonator 5 through a directional coupler comprising a plurality of slits distributed to the directional coupler 24 symmetrically with respect to the central axis of the resonator 5. The waveguides 6a and 7a each has a straightline formation without any folded portion as in the preceding embodiment. However, a tapered portion 35 curved in a circular arc form so as to pass through the central axis of the resonator 5 is provided for each waveguide at the central portion of the resonator 5, thus achieving the same filt'er function as the embodiment of FIG. 3.

It will be seen that various changes may be made in the above constructions of the embodiments without departing from the spirit and scope of the present invention and all matters contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. For example, instead of forming the tapered portions 35 in the waveguides 6a and 7a which differentiates the phase velocity of electromagnetic wave outside and inside of the resonator 5 in order to excite the resonator for transverse magnetic mode TM the insertion of dielectric materials in the waveguides 6a and 7a may achieve the same purpose. Also, the shape and arrangement of the coupling holes and slits 24 and 25 shown in FIGS. 3 and 4 may be modified in various ways so far as no unwanted or spurious mode is excited in the resonator 5. Further, it is apparent that although in FIGS. 3 and 4 the waveguides are coupled with the resonator having wider face plates to obtain transverse magnetic mode TM it is alternatively possible to obtain transverse electric mode TE by coupling the waveguides with the resonator having narrower face plates. Furthermore, the single resonator used in the abovedescribed embodiments may be replaced by a plurality of resonators which are mounted on one another and are coupled with one another by means of directional couplers each comprising a plurality of coupling holes or slits. Such a multi-stage arrangement of resonators may be made with higher mechanical stability because of the use of the resonators whose cross-sectional areas are not annular as in the conventional resonator but circular. An example of a two-stage arrangement of resonators is shown in FIG. 7 in which resonators 5a and 5b are coupled with each other by a directional coupler comprising a plurality of coupling holes 39. In FIG. 7, similar reference numerals are used to designate components similar to those of FIG. 3.

What we claim is:

1. A directional filter including a primary waveguide; a secondary waveguide, each of said waveguides having a rectangular cross-sectional area and a pair of wider faces plates and a pair of narrower face plates; resonator means having a circular cross-sectional area and opposite first and second circular end plates which have their radii substantially equal to the widths of said wider faces plates of said primary and secondary waveguides, a part of one of said wider faces plates of said primary waveguide and a part of one of said wider face plates of said secondary waveguide being coupled on and at least coextensive with at least a portion of said first end plate and at least a portion of said second end plate of said resonator means to form therein a first coupling area and a second coupling area, respectively, the direction of the center lines of said primary and secondary waveguides corresponding to the circumferential directions of said first and second end plates of said resonator means in said first andsecond coupling areas, respectively, said resonator means resonating at transverse magnetic mode TM (N=1,2, a first directional coupler comprising a plurality of first coupling elements extending in said first coupling area radially about the center of said first end plate of said resonator means for coupling said primary waveguide and said resonator means; and a second directional coupler comprising a plurality of second coupling elements extending in said second coupling area radially about the center of said second end plate of said resonator means for coupling said secondary waveguide and said resonator means.

2. A directional filter according to claim 1, wherein said resonator means comprises a plurality of adjacent resonators mounted on one another and a third directional coupler comprising a plurality of third coupling elements is disposed in an interface between the adjacent resonators, said third coupling elements extending radially about the center of said interface for coupling said adjacent resonators.-

3. A directional filter including a primary waveguide; a secondary waveguide, each of said waveguides having a rectangular cross-sectional area and a pair of wider face plates and a pair of narrower face plates; resonator means having a circular cross-sectional area and opposite first and second circular end plates which have their radii substantially equal to the widths of said narrower face plates of said primary and secondary waveguides, apart of one of said narrower face plate of said primary waveguide and a part of one of said narrower face plates of said secondary waveguide being coupled on and at least coextensive with at least a portion of said first end plate and at least a portion of said second end plate of said resonator means to form therein a first coupling area and a second coupling area, respectively,

the directions of the centerlines of said primary and secondary waveguides corresponding to the circumferential directions of said first and second end plates of said resonator means in said first and second coupling areas, respectively, said resonator means resonating at transverse electric mode TE, (N=l, 2,. a first directional coupler comprising a plurality of first coupling elements extending in said first coupling area radially about the center of said first end plate of said resonator means for coupling said primary waveguide and said resonator means; and a second directional coupler comprising a plurality of second coupling elements extending in said second coupling area radially about the center of said second end plate of said resonator means for coupling said secondary waveguide and said resonator means.

4. A directional filter according to claim 3, wherein said resonator means comprises a plurality of adjacent resonators mounted on one another and a third directional coupler comprising a plurality of third coupling elements is disposed in an interface between the adja cent resonators, said third coupling elements extending radially about the center of said interface for coupling said adjacent resonators. 

1. A directional filter including a primary waveguide; a secondary waveguide, each of said waveguides having a rectangular cross-sectional area and a pair of wider faces plates and a pair of narrower face plates; resonator means having a circular crosssectional area and opposite first and second circular end plates which have their radii substantially equal to the widths of said wider faces plates of said primary and secondary waveguides, a part of one of said wider faces plates of said primary waveguide and a part of one of said wider face plates of said secondary waveguide being coupled on and at least coextensive with at least a portion of said first end plate and at least a portion of said second end plate of said resonator means to form therein a first coupling area and a second coupling area, respectively, the direction of the center lines of said primary and secondary waveguides corresponding to the circumferential directions of said first and second end plates of said resonator means in said first and second coupling areas, respectively, said resonator means resonating at transverse magnetic mode TMN10 (N 1,2, . . .); a first directional coupler comprising a plurality of first coupling elements extending in said first coupling area radially about the center of said first end plate of said resonator means for coupling said primary waveguide and said resonator means; and a second directional coupler comprising a plurality of second coupling elements extending in said second coupling area radially about the center of said second end plate of said resonator means for coupling said secondary waveguide and said resonator means.
 2. A directional filter according to claim 1, wherein said resonator means comprises a plurality of adjacent resonators mounted on one another and a third directional coupler comprising a plurality of third coupling elements is disposed in an interface between the adjacent resonators, said third coupling elements extending radially about the center of said interface for coupling said adjacent resonators.
 3. A directional filter including a primary waveguide; a secondary waveguide, each of said waveguides having a rectangular cross-sectional area and a pair of wider face plates and a pair of narrower face plates; resonator means having a circular cross-sectional area and opposite first and second circular end plates which have their radii substantially equal to the widths of said narrower face plates of said primary and secondary waveguides, a part of one of said narrower face plate of said primary waveguide and a part of one of said narrower face plates of said secondary waveguide being coupled on and at least coextensive with at least a portion of said first end plate and at least a portion of said second end plate of said resonator means to form therein a first coupling area and a second coupling area, respectively, the directions of the centerlines of said primary and secondary waveguides corresponding to the circumferential directions of said first and second end plates of said resonator means in said first and second coupling areas, respectively, said resonator means resonating at transverse electric mode TEN11(N 1, 2,. . .); a first directional coupler comprising a plurality of first coupling elements extending in said first coupling area radially about the center of said first end plate of said resonator means for coupling said primary waveguide and said resonator means; and a second directional coupler comprising a plurality of second coupling elements extending in said second coupling area radially about the center of said second end plate of said resonator means for coupling said secondary waveguide and said resonator means.
 4. A directional filter according to claim 3, wherein said resonator means comprises a plurality of adjacent resonators mounted on one another and a third directional coupler comprising a plurality of third coupling elements is disposed in an interface between the adjacent resonators, said third coupling elements extending radially about the center of said interface for coupling said adjacent resonators. 